Sensors are commonly used to detect the presence or absence of a particular chemical. Known sensors have been provided in a variety of forms for achieving this purpose. For example, signs and badges have been developed—see, for example, U.S. Pat. No. 6,284,198 and U.S. patent application Ser. No. 2004/0223876A1—along with filters, filter cartridges, and breathing masks (respirators) that include chemical indicators—see, for example, U.S. Pat. Nos. 5,323,774, 5,297,544, and 4,684,308. One particular chemical sensor that has been developed is a passive end-of-service-life indicator (ESLI) that has a film-like body. Within the film body is a detection layer that detects the presence of a particular chemical. Examples of film-like passive ESLIs are described in U.S. Patent Publications 2008/0063575A1 and 2008/0063874A1 to Rakow et al. ESLIs may be tailored to respond to various organic vapors and reactive gases.
ESLIs have been used in filter cartridges to assist in informing the user of when the cartridge has met the end of its service life—see, for example, U.S. Pat. Nos. 7,442,237, 6,497,756, 5,323,774, 5,297,544, 4,684,380, 4,530,706, 4,365,627, 4,326,514, and 4,154,586. U.S. Patent Application 2007/0137491 and Ser. Nos. 12/470,865, 12/470,890, and 12/470,920 also describe filter cartridges that use end-of-service-life indicators. The ESLIs are disposed adjacent to the housing sidewall so that they can be easily viewed from the exterior. The ESLIs are generally designed to be colorimetric sensors—that is they change color after being exposed to a sufficient amount of contaminant in the filtered air. The color change and sensor placement are tailored to provide indication that coincides with when the filter cartridge service life ends. One particular problem with colorimetric ESLIs is that the wearer needs to memorize the original color to know when a color change has occurred if an original reference color is not available adjacent the ESLI. If the wearer fails to do this, the wearer will not be certain whether the ESLI is displaying the original color or the warning color. Because known ESLIs are not fashioned to provide a warning to the user, which warning does not require the user to memorize or distinguish between the original color and the warning color, safety issues may arise if the wearer fails to notice the color change.